Manage AI Buddy topic grouping and review the sources used per question

Context / Problem: The "Top topics" overview shows which subjects employees ask AI Buddy about most, but the grouping is generated automatically and cannot be adjusted. Admins also cannot see which documents were used to answer the questions within a topic, so there is no way to check whether the answers given were correct.

Current behavior:
- Topics in "Top topics" are grouped automatically, with no way to correct or reorganise the grouping
- The overview shows only the number of questions per topic
- The sources used for those answers are not shown, so answers cannot be reviewed

Desired solution:
- Manage topics themselves: create, rename, merge, and split topics, and decide which questions belong to which topic

- Open a topic to see the underlying questions that were asked
- See, per question, which document(s) the answer was based on
- Open that source document directly from the overview to verify the answer